Adjusting what fstypes df displays

[Bryce Harrington]

These days, df displays a lot of mount points, due to the increased use
of non-consumable filesystems such as tmpfs and squashfs.  This clutter
is particularly noticeable using df in Ubuntu, due to the increased
popularity of snaps, but the general problem affects all Linux distros,
and shows up in other commands such as lsblk, blkid, fdisk -l, and
mount.

A commonly documented user customization[1] is to use aliases, e.g.:

    alias df='df -h -x squashfs -x tmpfs -x devtmpfs'
    alias lsblk='lsblk -e 7'

df's upstream is aware of the issue, and would like to address it in
their code.  They've considered a number of solutions[2], but do not
appear to have come to a consensus as of yet.  Among the raised concerns
are variations in distro requirements, and providing an ability for
users to override/customize the exclusions.

A solution I am considering to propose would add an environment
variable, DF_EXCLUDE_FSTYPES, that df would treat similar to the -x
flag.


I've drafted a POC implementation for df here:

  https://github.com/bryceharrington/coreutils/commit/cc372d778b44abcf81af42743a5174685f9bb4db


Here's what it does inside a container...

Before:

    $ df
    Filesystem                   1K-blocks      Used Available Use% Mounted on
    default/containers/coreutils  24891008   2258816  22632192  10% /
    none                               492         4       488   1% /dev
    udev                          32862724         0  32862724   0% /dev/tty
    tmpfs                              100         0       100   0% /dev/lxd
    /dev/nvme0n1p2               982940092 348781368 584158392  38% /home/bryce/pkg
    tmpfs                              100         0       100   0% /dev/.lxd-mounts
    tmpfs                         32892912         0  32892912   0% /dev/shm
    tmpfs                          6578584       224   6578360   1% /run
    tmpfs                             5120         0      5120   0% /run/lock
    tmpfs                         32892912         0  32892912   0% /sys/fs/cgroup
    tmpfs                          6578580         0   6578580   0% /run/user/1001

After:

    $ export DF_EXCLUDE_FSTYPES=tmpfs,devtmpfs,squashfs

    $ df
    Filesystem                   1K-blocks      Used Available Use% Mounted on
    default/containers/coreutils  24891008   2258816  22632192  10% /
    /dev/nvme0n1p2               982940092 348781372 584158388  38% /home/bryce/pkg

(It's even more drastic on my desktop - from 39 lines down to 6.)

For Ubuntu, DF_EXCLUDE_FSTYPES could be set in the global profile (under
/etc/profile.d/ perhaps?)  This would make it straightforward to add
new non-consumable filesystems that may crop up down the road.  Perhaps
other tools could use a similar/same env var approach, giving us a
uniform way to control fs listings in the distro.

Two alternate approaches I'm weighing are a config file in /etc, or just
a package patch to carry in coreutils, but the env var approach feels
like it would be more flexible to users and hopefully more suitable for
upstream.  Before I forward it upstream, though, what does ubuntu-devel@
think?

Thanks,
Bryce



1:  Google shows many links, here's a sample:

    https://bugs.launchpad.net/ubuntu/+source/apt/+bug/1756595
    https://askubuntu.com/questions/1029493/how-can-i-stop-snaps-from-listing-in-df
    https://clintonminton.com/how-to-clean-up-snap-and-loop-devices-from-df-output-in-ubuntu-18-04/

    Note that one drawback to this approach is it's not overrideable, i.e.:

    $ alias df='df -h -x squashfs -x tmpfs -x devtmpfs'
    $ df -t tmpfs
    df: file system type ‘tmpfs’ both selected and excluded

2: Ideas the maintainers discussed include omitting read-only devices,
    or ones with non-consumable storage, or with <1% usage.  They also
    discussed maintaining a simple list of excludable "lesser" file
    systems, possibly stored in a user's ~/.dfrc or ~/.shellrc.

    https://debbugs.gnu.org/cgi/bugreport.cgi?bug=37702

3:  We exclude squashfs,tmpfs,devtmpfs in our nagios packaging, where
    these were generating false-positive 'disk full' alerts:

    https://bugs.launchpad.net/charm-nagios/+bug/1827159